Effusion cells for molecular beam epitaxy apparatus

ABSTRACT

The effusion cell for molecular beam epitaxy apparatus consists of a pyrolytic BN cylindrical crucible surrounded by a heating coil. The position of the heating coil is maintained by a plurality of ceramic rods extending parallel to the cylinder axis with notches along their length for engaging the heating coil. The ceramic rods are secured by a retaining ring near the front of the crucible and an apertured disk near the back of the crucible. The entire assembly is surrounded by a foil heat shield.

This application is a division of application Ser. No. 755,663, filedDec. 30, 1976, now U.S. Pat. No. 4,137,865, and was concurrently filedwith another division entitled "Molecular Beam Method for Processing aPlurality of Substrates."

BACKGROUND OF THE INVENTION

This invention relates to the molecular beam deposition of semiconductormaterials under ultra-high vacuum conditions, and, more particularly, tothe sequential deposition of such materials on a plurality ofsubstrates, and to unique effusion cells for use in the process.

As pointed out in a review paper by J. R. Arthur and myself (Progress inSolid-State Chemistry, Vol. 10, Part 3, pp. 157-191, Pergamon Press1975), molecular beam epitaxy (MBE) is a term used to denote theepitaxial growth of semiconductor films by a process involving thereaction of one or more thermal molecular beams with a crystallinesurface under ultra-high vacuum conditions. In comparison to simplevacuum-evaporation, MBE offers much improved control over the incidentatomic or molecular fluxes so that differences between the stickingcoefficients of the beam species can be taken into account. Use ofshutter mechanisms and relatively slow growth rates (e.g., 1 μm/hr.)allow rapid changing of beam species and growth of layers as thin as amonolayer (2.8 A for GaAs).

In addition, since electrically active impurities are added to thegrowing film by means of separate beams, the doping profile normal tothe surface can be varied and controlled with a spatial resolutiondifficult to achieve by more conventional, faster growth techniques suchas CVD and LPE.

MBE has been used to fabricate films of a variety of materials: GroupIII-V compounds, principally GaAs and A1GaAs, as well as Group II-VI andIV-VI materials (e.g., CdTe and PbS). Research also has extended toelemental materials such as Si but to date has met with, at best, mixedresults. Molecular beam deposition is not limited, however, to epitaxialgrowth on single crystal substrates-high resistivity, polycrystallineGaAs layers have been deposited on amorphous substrates such as SiO₂.

In the GaAs-AlGaAs system MBE has been successfully employed tofabricate a number of device structures: IMPATT diodes, microwave mixerdiodes, double heterostructure junction lasers, optical waveguides andsuperlattices. Fabrication took place, however, in either a researchenvironment or a low-volume production facility so that processing of asingle substrate (which can typically accommodate hundreds of devices)at a time was adequate.

It is, however, a broad object of my invention to sequentially process aplurality of substrates in an MBE apparatus suitable for relatively highvolume production.

SUMMARY OF THE INVENTION

In accordance with an illustrative embodiment of my invention, molecularbeam apparatus for processing a plurality of substrates comprises anevacuable growth chamber including a plurality of ovens for generatingmolecular beams directed to a growth position therein, an evacuableauxiliary (sample-exchange) chamber, an air lock which either isolatesthe chambers or places them in communication with one another, andcarrier means for moving the substrates between the chambers andsequentially to the growth position, CHARACTERIZED IN THAT: the growthchamber includes (1) a cryogenic cooling shroud which surrounds all ofthe substrates in the growth chamber and which has an aperture to admitthe molecular beams to the growth position, and (2) means forselectively heating the substrates so that only the substrate in thegrowth position (the growth substrate) is heated--the other (idle)substrates being unheated. The shroud configuration reducescontamination of the idle substrates by confining the beam spread viathe aperture and by placing the idle substrates in a cryogenicsurrounding so that reflected species of the beams are adsorbed by theshroud. On the other hand, leaving idle substrates unheated reduces theevaporation of high vapor pressure elements from those substrates.

In another embodiment, my invention is further characterized in that:the auxiliary chamber includes (3) a port which permits access to thesubstrates and (4) means for maintaining a positive pressure therein(with respect to the ambient) when the port is open. Preferably an inertgas such as dry nitrogen or argon is pumped into the auxiliary chamberto create the positive pressure. These features enable the substrates tobe exchanged, inspected, etc. while the air lock is closed withoutadmitting ambient contamination (e.g., water vapor, hydrocarbons) intothe auxiliary chamber and hence onto the processed substrates, andwithout having to remove the entire carrier mechanism. This mechanism,which is illustratively a rod-bellows arrangement secured to a portcover, tends to be heavy and bulky and thus difficult to handle.

A third aspect of my invention is the provision of a unique pyrolytic BN(boron nitride) effusion cell which can be used in eithersingle-substrate or multiple-substrate systems. The cell comprises anelongated, cylindrical, pyrolytic BN crucible surrounded by a heatingcoil. A plurality of ceramic rods, notched along their length, hold thewires in place againt the crucible. Knurled foil surrounds the entireassembly as a radiation heat shield.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects of my invention, together with its variousfeatures and advantages, can be readily understood from the followingmore detailed description taken in conjunction with the accompanyingdrawing, in which:

FIG. 1 is a schematic end view of molecular beam apparatus in accordancewith one embodiment of my invention;

FIG. 2 is a schematic side-view taken along line 2--2 of FIG. 1, but forsimplicity FIG. 2 is not shown to be precisely a cross-section of FIG.1;

FIG. 3 is a cross-sectional view of an effusion cell in accordance withanother aspect of my invention; and

FIG. 4 is a view taken along line 4--4 of FIG. 3.

DETAILED DESCRIPTION Apparatus

With reference now to FIGS. 1 and 2, there is shown ultra-high vacuumapparatus 10 for the molecular beam deposition of layers ofsemiconductor materials sequentially on a plurality of substrates11.1-11.4. The apparatus 10, which is typically made of stainless steel,comprises an evacuable growth chamber 12, an auxiliary (sample-exchange)chamber 14 and means, such as air lock 16, for alternately isolating thechambers from one another and placing them in communication with oneanother. Air lock 16 serves these functions by means of channel 16.1which extends between the two chambers in conjunction with a gate 16.2which can be moved (by means not shown) into place to block the channel16.1. When so placed, gate 16.2 is pressed against 0-ring 16.3 to form avacuum seal, thus isolating the two chambers. Isolation is furtherenhanced by the fact that a higher vacuum is maintained in growthchamber 12 when the gate 16.2 is closed which tends to draw the gateagainst the 0-ring. As will become apparent hereinafter, thisconfiguration of coupled chambers means that growth chamber 12 need notbe pumped down from atmospheric pressure to a high vacuum (e.g., 10⁻⁸Torr) after deposition on each set of substrates is completed, therebyreducing both the system down-time as well as contamination of thegrowth chamber.

Vacuum conditions are achieved and maintained in the growth chamber 12by suitable pumping means, typically a Ti sublimation pump 18 coupled toa commercially available vacuum pump (not shown), preferably an oil-freeone such as an ion pump. The sublimation pump 18 includes a Ti element18.1 positioned within a cryogenically cooled shroud 18.2. A pluralityof spaced baffles 18.3 block line-of-sight paths between element 18.1and the substrates. A cryogenic fluid, such as liquid nitrogen (LN₂), ispumped through the hollow walls of shroud 18.2 via inlet 18.4 whichextends through port cover 20 to the exterior.

In the auxiliary chamber 14, on the other hand, the pumping means isconsiderably simpler. A sublimation pump is not required. Rather,another commercially available oil-free vacuum pump is coupled to thechamber 14 via port 22. Of course, it is apparent that this pump and theone used to pump the growth chamber 12 can be one and the same, in whichcase suitable valves would be employed to switch the pump from onechamber to another (or to both).

The auxiliary chamber 14 also includes an exchange port 24 which permitsaccess to the interior thereof, especially when gate 16.2 is closed toisolate the growth chamber from the auxiliary chamber. This port allowsthe substrates to be exchanged for fresh substrates after growth iscompleted, or after some intermediate point in the growth sequence whichis to be followed by further processing (e.g., etching, oxidedeposition, masking) in another facility. In addition, to preventcontamination (principally by water vapor and hydrocarbons) of freshsubstrates prior to layer growth, means is provided for maintaining theauxiliary chamber 14 under a positive pressure (with respect to theexterior or ambient). This means is depicted by inlet 26 through which anoncorrosive gas such as dry nitrogen or argon is pumped from acommercially available source (not shown).

Within the auxiliary chamber 14 carrier means 30 moves the substrates11.1 to 11.4 between the auxiliary and growth chambers. Means 30illustratively comprises a rod-bellows mechanism in which each substrateis mounted on a holder 32 which in turn is secured to a rod 34. AU-shaped retainer or guide 36 supports the rod 34 and itself is securedto the cap 38.1 of port 38. A bellows 40 is secured between a base plate34.1 at the end of rod 34 and the same cap 38.1, thereby forming avacuum seal so that the interior 40.1 of the bellows is essentially atatmospheric pressure but the exterior is at the auxiliary chamberpressure.

In order to translate rod 34 without breaking the vacuum, the side ofbase plate 34.1 interior to the bellows 40 is provided with an aperturedsupport member 34.2. A second rod 42 is threaded through port 38 and isattached to a handle 44 (or to suitable automatic means for rotation)exterior to the chamber. The other end of rod 42 terminates in a secondbase plate 42.1 interior to support member 34.2. In order to facilitaterotation base plate 42.1 is separated from base plate 34.1 and theapertured wall of support member 34.2 by suitable friction-reducingmeans such as ball-bearings 34.3.

As mentioned previously, the substrates are mounted on suitable holders32. In FIG. 1, substrate 11.1, for example, is indium soldered to amolybdenum slab 32.1 which in turn is secured to a molybdenum heatingblock 32.2. Block 32.2 is then mounted on rod 34. Each block 32.2 has apair of electrical contacts 32.3 depicted by notches in FIG. 2 so thatthe substrates can be selectively heated to the growth temperature.

Selectively heating is achieved by means of a second, but simpler,bellows mechanism 60 in which a pair of elongated contact fingers 60.1are carried by base plate 60.2. The fingers 60.1 are translated throughaperture 46.2 in shroud 46 by rod 60.3 which is threaded through cap60.4 and is rotatably mounted to base plate 60.2. As before a bellows60.5 is attached to cap 60.4 and plate 60.2 to form a vacuum seal. Byselectively heating only the growth substrate (i.e., the substrate inthe growth position), surface damage caused by evaporation of high vaporpressure elements from compound materials (e.g., As from GaAs) isvirtually eliminated.

Although two contact fingers are shown, it is apparent that, if thesystem were suitably grounded electrically, a single contact would beadequate.

Within the growth chamber 12, each substrate is translated past a growthposition defined by an aperture 46.1 in a second LN₂ shroud 46. Thisaperture is aligned to face a plurality of heatable ovens 48 from whichthe molecular beams emanate when shutters 48.1 are opened. These ovens(i.e., guns, Knudsen cells) are surrounded by a third LN₂ shroud 50within oven port 52. Preferably, ovens 48 are positioned so that none isoriented either at an angle above horizontal (to avoid having sourcematerial therein fall out) or at an angle near vertical (to reduce"popping" or bursting of high vapor pressure components of compoundsemiconductor materials, e.g., As bursting through GaAs covered with Ga,and to reduce the likelihood that beam species which adhere to theshutters will fall back into the ovens).

A unique effusion oven in accordance with another aspect of my inventionis schematically shown in FIGS. 3 and 4. It comprises an elongated,cylindrical crucible 48.2 typically made of a high purity, refractorymaterial such as pyrolytic BN. One end is open to permit egress of amolecular beam evaporated from source material 49. The crucible 48.2 isheated with a spiral heater winding 48.3 (e.g., Ta or W wire) which isfirmly pressed onto the crucible by a set of ceramic (e.g., Al₂ O₃) rods48.4 notched along their length. The rods are secured by a retainingring 48.8 at the front and an apertured disk 48.9 at the back. The wholeassembly is wrapped a plurality of times with layers of knurled foil48.5 (e.g., Ta, W) which act as a radiation heat shield. The temperatureof the effusion oven is measured by a pair of thermocouples 48.6inserted in a dimple 48.7 formed on the closed end of the crucible 48.2.

This type of effusion oven is useful with a variety of source materials,especially Al which tends to crack crucibles made of graphite.

it is important to note that shroud 46 is at least co-extensive with thezone defined by all of the substrates when they are simultaneously inthe growth chamber 12. That is, shroud 46 surrounds both the growthsubstrate (e.g., substrate 11.1) as well as the idle substrates (e.g.,11.1-11.3) in the growth chamber. Thus, molecular beam species which arereflected from the growth substrate (or heating block) tend to adhere tothe cryogenically cooled shroud 46 rather than continuing to bereflected within the chamber. This configuration in conjunction withaperture 46.1, which tends to confine the beams to the growth substrate,cooperates to reduce contamination of the idle substrates.

The growth chamber 12 has a number of other ports adapted to accommodatea variety of equipment used to monitor growth and vacuum conditions. inFIG. 2, ion guage 62 monitors the background pressure. In FIG. 1, on theother hand, ion guage 64 is used to monitor the beam flux. Sputter iongun 66 is optional and is used to clean the substrate surface.Generally, the native oxide technique mentioned hereinafter issufficient, but if for some reason the substrate is subsequentlycontaminated, then sputtering can be used to produce an atomically cleangrowth surface. View port 68 is also optional and of course permitsvisual observation of the substrates. In each case, shroud 46 hassuitable apertures so that a line-of-sight path is established betweenthe substrate and the particular equipment.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

In particular, instead of translating the rods through the growthposition by the rod-bellows mechanism of FIG. 2, it is also possible tocarry the substrates on a spoke-like apparatus; i.e., the spokes emanatefrom a rod, the substrates are mounted on the ends of the spokes, andthe rod is rotated to sequentially place the substrates in the growthposition.

Either type of mechanism for moving substrates to the growth positioncould readily be provided with a lock-in or stepping feature whichautomatically stops successive substrates at the growth position.

PROCESS STEPS

In operation of the above-described apparatus it will be assumedinitially that both the growth chamber and the auxiliary chamber are atatmospheric pressure. The following then is a typical sequence ofprocess steps utilized to fabricate epitaxial layers of GaAs on GaAssubstrates.

(1) The substrates are first prepared to remove surface contaminationpreferably by the native oxide process described in my U.S. Pat. No.3,969,164.

(2) The substrates are then indium soldered to molybdenum slabs andmounted through exchange port 24 onto the rod 34.

(3) One of the ovens 48 is filled with liquid Ga and the other withpolycrystalline GaAs (primarily to provide a source of arsenic).Alternatively, arsenic can be supplied from a gaseous state by means ofa separate container located exterior to the chamber. The other ovensare optional and can be used, for example, as a source of Al to growAlGaAs or as a source of Sn and Mg to produce n- and p-type conductivityrespectively in the GaAs or AlGaAs layers.

(4) With gate 16.2 open and exchange port 24 closed, an oil-freesorption pump is utilized to reduce the pressure in both chambers toabout 10⁻³ and 10⁻⁴ Torr. This step typically takes about half an hour.

(5) Next, an ion pump, or the equivalent, is used to reduce the pressureof both chambers to about 10⁻⁷ to 10⁻⁸ Torr. This step typically takes afew hours. It should be noted that the ion pump remains on at all timesin the growth chamber, but when gate 16.2 is closed it may be turned offin the auxiliary chamber.

(6) The titanium sublimation pump is then flashed so that titanium isdeposited on the interior surface of shroud 18.2, thereby reducing thepressure of both chambers from about 10⁻⁸ to 10⁻⁹ Torr.

(7) The ovens are actuated by heating them to a temperature which is10°-20° C. above the oven temperature utilized for growth. For example,the Ga gun would be heated to about 995° C. and the As gun would beheated to about 360° C.

Steps 4 through 7 are typically referred to as the pump-down process andare generally not repeated unless for some reason the growth chamber hasbeen brought to atmospheric pressure. The following sequence of steps istermed the growth process.

(8) The rod-bellows mechanism is activated to move the substrates fromthe auxiliary chamber into the growth chamber.

(9) Optionally argon gas can be introduced into the growth chamber to apressure of about 10⁻⁴ Torr and the sputter ion gun 66 can be actuatedfor about one-half to one hour in order to further clean the substratesurfaces. The argon gas is then pumped out of the chambers.

(10) The shrouds 46 and 50 are filled with liquid nitrogen which tendsto trap contaminants on the cooled shroud surfaces, thereby reducing thepressure of both chambers to about 10⁻⁹ Torr.

(11) With the shutters 48.1 closed the ovens 48 are then lowered intemperature to the growth temperature.

(12) Next, the titanium sublimation pump is again flashed.

(13) The rod-bellows mechanism 60 is then actuated in order to bringcontact fingers 60.1 into electrical contact with the heating blocks inthe growth position. A voltage applied across contacts 60.2 causescurrent to flow through fingers 60.1 and a heating element (not shown)within the block, thereby heating the growth substrate to the growthtemperature (e.g. 450°-650° C., typically 560° C.). At this point itshould be noted that the background pressure in the growth chamber isabout 10⁻⁶ Torr and is primarily caused by the presence of arsenic fromthe heated ovens. This background pressure is advantageous in reducingsurface damage of the growth substrate which would be caused by theevaporation of arsenic therefrom. Because the idle substrates are notheated to the growth temperature, however, such evaporation from them isinsignificant.

(14) In order to effect layer growth, the shutters are opened so that Gaand As beams are directed through aperture 46.1 in shroud 46 to thesubstrate in the growth position (e.g. substrate 11.1). Growth continuesfor a time period effective to grow a layer of the desired thickness. Atypical growth rate is about 1 μm/hour.

(15) Each substrate is then sequentially moved into the growth positionand steps 13 and 14 are repeated to effect layer growth on each.

(16) When growth on each of the substrates has been completed, theshutters 48.1 are closed and the temperature of the substrates as wellas the ovens is lowered.

(17) Then the completed substrates are translated back into theauxiliary chamber by means of the rod-bellows mechanism.

(18) With the rod 34 and attached substrates fully retracted into theauxiliary chamber 14, the gate 16.2 is closed.

(19) Next, the auxiliary chamber is refilled with dry nitrogen andexchange port 24 is open.

(20) The substrates can now be removed from the auxiliary chamber (i.e.,the substrates attached to the molybdenum slabs 32.1 are detached fromthe heating blocks 32.2) and fresh substrates can be mounted on theholders.

(21) The exchange port 24 is closed and the auxiliary chamber is pumpedto a pressure of about 10⁻⁸ to 10⁻⁹ Torr.

(22) The gate 16.2 can now be opened and steps 8-21 repeated to effectgrowth of epitaxial layers on the fresh substrates. As mentionedpreviousy, this process provides a relatively low system down-timebecause the growth chamber 12 is isolated from the auxiliary chamberduring substrate exchange. Consequently, the growth chamber is notexposed to the atmosphere and the ovens need not be out-gassed again.

What is claimed is:
 1. A molecular beam epitaxy apparatus comprising anevacuated growth chamber and at least one improved effusion cell forgenerating a molecular beam from a source material, said improvedeffusion cell comprising:an elongated cylindrical Pyrolytic BoronNitride crucible having an opening at one end thereof to permit egressof said beam, a heating coil surrounding and in heat transferrelationship with said crucible, a plurality of ceramic rods, extendingessentially parallel to the cylinder axis and having notches along theirlength, for engaging said coil and firmly pressing said coil onto theouter surface of said crucible; a retaining ring to secure said ceramicrods near said one end of the crucible, an apertured disc to secure saidceramic rods near the other end of the crucible; and a radiation heatshield surrounding the crucible, coil and rod assembly.